San Jose, CA, United States
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Spurlock F.,California Dep. of Pesticide Regulation | Simunek J.,University of California at Riverside | Johnson B.,California Dep. of Pesticide Regulation | Tuli A.,California Dep. of Pesticide Regulation
Vadose Zone Journal | Year: 2013

We conducted a comprehensive Monte Carlo sensitivity analysis of fumigant transport and volatilization using HYDRUS. The sensitivities of fumigant flux and soil-gas concentrations varied by application scenario and are interpreted mechanistically. These data provide a reference for designing fumigation field studies and interpreting their results. Interest in the use of vadose zone transport models for fumigant risk assessment is increasing. Good modeling practice includes an assessment of model sensitivity and output uncertainty. This computational study evaluated the sensitivity of HYDRUS-1D- and HYDRUS 2D/3D-simulated fumigant cumulative flux, maximum 6-h period mean flux density, and soil gas concentrations to 15 model input variables using Monte Carlo Latin hypercube analysis. The input variables included fumigant physicochemical properties, agricultural film (tarp) properties, and soil properties. Three different application scenarios were investigated: tarped broadcast, tarped bedded shank injection, and a tarped drip line-source application. Model sensitivity to initial water content (θi), saturated water content (θs), and tarp permeability varied among scenarios depending on the relative importance of soil gas diffusive resistance and tarp mass transfer resistance to fumigant volatilization. Model outputs were sensitive to fumigant air-water partition and gas-phase diffusion coefficients, two parameters that probably have a small contribution to overall modeling uncertainty because accurate estimation methods for these parameters are available. Sensitivities to the fumigant degradation rate were high in all scenarios, and sensitivity to tarp permeability was high only when substantial volatilization occurred from the tarped portion of soil surfaces. Existing literature data for both degradation and tarp permeability are highly variable; parameterizing these processes using literature estimates may contribute substantially to model uncertainty. In several cases, the highest output sensitivities were to θs. For model comparisons to site-specific field data, soil texture-based estimates of θs are potentially large contributors to model uncertainty; direct measurement is recommended. © Soil Science Society of America, 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved.


Zhang X.,California Dep. of Pesticide Regulation | Goh K.S.,California Dep. of Pesticide Regulation
Journal of Environmental Quality | Year: 2015

Three models were evaluated for their accuracy in simulating pesticide runoff at the edge of agricultural fields: Pesticide Root Zone Model (PRZM), Root Zone Water Quality Model (RZWQM), and OpusCZ. Modeling results on runoff volume, sediment erosion, and pesticide loss were compared with measurements taken from field studies. Models were also compared on their theoretical foundations and ease of use. For runoff events generated by sprinkler irrigation and rainfall, all models performed equally well with small errors in simulating water, sediment, and pesticide runoff. The mean absolute percentage errors (MAPEs) were between 3 and 161%. For flood irrigation, OpusCZ simulated runoff and pesticide mass with the highest accuracy, followed by RZWQM and PRZM, likely owning to its unique hydrological algorithm for runoff simulations during flood irrigation. Simulation results from cold model runs by OpusCZ and RZWQM using measured values for model inputs matched closely to the observed values. The MAPE ranged from 28 to 384 and 42 to 168% for OpusCZ and RZWQM, respectively. These satisfactory model outputs showed the models' abilities in mimicking reality. Theoretical evaluations indicated that OpusCZ and RZWQM use mechanistic approaches for hydrology simulation, output data on a subdaily time-step, and were able to simulate management practices and subsurface flow via tile drainage. In contrast, PRZM operates at daily time-step and simulates surface runoff using the USDA Soil Conservation Service's curve number method. Among the three models, OpusCZ and RZWQM were suitable for simulating pesticide runoff in semiarid areas where agriculture is heavily dependent on irrigation. © 2015 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved.


Budd R.,California Dep. of Pesticide Regulation | Ensminger M.,California Dep. of Pesticide Regulation | Wang D.,California Dep. of Pesticide Regulation | Goh K.S.,California Dep. of Pesticide Regulation
Journal of Environmental Quality | Year: 2015

The phenylpyrazole insecticide fipronil has become a popular replacement pest management tool as organophosphorus insecticides have been phased out for residential use and pyrethroids have come under scrutiny as a surface water contaminant. There has been an increasing concern of offsite transport of fipronil to surrounding surface waters and a corresponding increase in potential toxicity to aquatic organisms. The California Department of Pesticide Regulation Environmental Monitoring Program has collected over 500 urban surface water samples throughout California since 2008 to determine the presence and concentrations of fipronil and five degradate products. Statewide, fipronil was detected at high frequency (49%), as were the sulfone (43%) and desulfinyl (33%) degradates. Data collected at long-term monitoring stations indicate higher concentrations in southern California, corresponding to a higher use pattern in the region. There is a clear pattern of increased transport of fipronil with higher flow associated with rain events. However, the lack of seasonality effects on degradates' concentrations suggest a constant source of fipronil with a corresponding lag time of transport to surface waters during the dry season. Receiving waters had a diluting effect on concentrations; however, a significant proportion (46%) of receiving water samples had associated fipronil concentrations above USEPA aquatic life chronic benchmark values. Total mass loading estimates from a long-term monitoring site suggest that the annual fipronil loading is greater in the dry season than during storm events. This could have implications for future mitigation efforts because most runoff during this period was generated from irrigation and outdoor residential use. © 2015 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.

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